1. Field of the Invention
The present invention relates to a method of detection of a boil dry condition of a utensil placed on a glass-ceramic cooking surface of a cooking unit, which has at least one cooking area, which is associated with a heating element, whose energy input is adjusted by means of a heating element power control device, and in which the temperature, or a variable dependent on it, such as a signal (temperature signal), is measured in operation and is limited by means of a temperature limiting protective device with an adjustable cutoff temperature.
The invention also relates to a device or apparatus for performing this method.
2. Prior Art
The moisture or liquid present in cooking material in a cooking utensil or vessel can be completely evaporated during cooking when too much heat energy is supplied or when it is heated for too long a time interval. This situation is called a boil dry condition. It can lead to damage of the cooking apparatus in the case of aqueous cooking materials, especially of the glass-ceramic cooking surface and the vessel. With other cooking materials there is a danger that the cooking material can ignite or burn and damage surrounding devices. In this case the energy supply is preferably turned off and the operator is at least warned. Typically this error occurs when high heat energy input used for a rapid cooking phase is not turned off because the operator has forgotten that high heat energy input is being used.
Of course the surface temperature of the cooking area on glass-ceramic cooking surfaces of cooking units is typically monitored and protected by means of rod-like mechanical temperature limiting protective devices. These devices measure the average temperature under the cooking area and limit it. The shutoff temperature, also its so-called threshold value, is established on the basis of the properties of the standard cooking vessels or utensils and the glass-ceramic material. Also an electronic device for limiting the temperature is known. However only the behavior of the mechanical temperature limiting protective devices is sufficient sensitive, so that an additional evaluation of temperature information is not required.
The boil dry condition of a cooking utensil cannot be detected only by means of the typical temperature limiting protective device provided with a glass-ceramic cooking surface of a cooking unit.
A method and corresponding device for detection of the so-called boil dry condition is disclosed in U.S. Pat. No. 4,493,981. In this method the first derivative of the temperature with respect to time is used as the indicator for this condition. When it exceeds a certain predetermined limiting value, then a boil dry condition is detected. The limiting value is dependent among other things on the type of cooking unit and the actual power set. Furthermore a boiled dry vessel or utensil is detected, when the temperature in the cooking vessel or utensil exceeds a certain limiting value. Also this value depends on the type of cooking unit and the actual input power. When a boil dry condition is detected, the operator is warned or the cooking unit is turned off.
In this known method it is disadvantageous that the detection of the critical operating condition depends on the actual input power. Also in practice this has the consequence that no uniform limit for the first time derivative of the temperature can be used to reliably detect the boil dry condition for different types of cooking utensils. More rapid increases in temperature can occur in filled poorly heat-conducting utensils than in empty, heat conductive utensils.
The boil dry condition is detected by measurement of the temperature in the interior and exterior of a two-circuit heating element in the device described in DE 40 22 846 C2. This known device is however limited to glass-ceramic cooking surfaces with two-circuit heating elements.
A method for detection of the boil dry condition of a vessel on a glass-ceramic cooking surface of a cooking unit and an associated apparatus are disclosed in DE 43 36 752 A1. In this method the temperature is measured and its first derivative is determined. When the value of the first derivative first exceeds a predetermined boundary or limiting value, the initial heating power is reduced by 100%. This reference also discloses that empty metal vessels being heated are detected because of the reduced electrical heating power required to reach the same temperature and the current supply is shut off when the input power drops below a certain predetermined value. This method has the same disadvantage as described above for the U.S. Patent reference.